Fodaro, if WoW accept images larger than 512px then I could do like that but otherwise it's gonna be hard fitting some parts into the backdrop.

Ah, good point, but is it that WoW won't accept image files bigger than 512x512, or textures bigger than 512x512?

I'm not sure which, but if it is the files after all, then everything will fit into 64x64 squares apart from the bottom corners. So, you can either change the size of the corners, or, since you don't appear to be using transparency, manually position two textures in the right place (shown by the red squares on the attachment) and use a border for the rest (shown by white squares).